1. Field of the Invention
The present invention relates to inkjet printing devices, and, in particular, to a method and apparatus for servicing a printhead.
2. Discussion of the Background Art
Inkjet printing mechanisms may be used in a variety of different printing devices, such as plotters, facsimile machines and inkjet printers, collectively referred to herein as printers. These printing mechanisms typically use a printhead to shoot drops of ink onto a page or sheet of print media. Some inkjet print mechanisms utilize a type of printhead called a cartridge that carries a self contained ink supply back and forth across the media. In the case of a multi-color cartridge, several printheads and reservoirs may be combined into a single unit. In this case, the multi-color cartridge is also referred to as a printhead.
Other inkjet print mechanisms, known as “off-axis” systems, propel only a small amount of ink in the printhead across the media, and include a main ink supply in a separate reservoir, which is located “off-axis” from the path of printhead travel. Typically, a flexible conduit or tubing is used to convey the ink from the reservoir to the printhead. A printhead may also have a cap or capping mechanism such that when the printhead is not printing, the printhead is covered. This may serve to prevent the printhead from drying and/or to otherwise protect the printhead from the environment.
Each printhead includes a series of nozzles through which the ink drops are fired. The particular ink ejection mechanism within the printhead may take on a variety of different forms known to those skilled in the art, such as those using piezo-electric or thermal printhead technology. For instance, two earlier thermal ink ejection mechanisms are shown in U.S. Pat. Nos. 5,278,584 and 4,683,481, both assigned to the present assignee, Hewlett Packard Company. In a thermal ejection system, a barrier layer containing ink channels and vaporization chambers is located between a nozzle orifice plate and a substrate layer. This substrate layer typically contains linear arrays of heater elements, such as resistors, which are energized to heat ink within the vaporization chambers. Upon heating, an ink droplet is ejected from a nozzle associated with the energized resistor.
To print an image, the printhead is scanned back and forth across above the media in an area known as a print zone, with the printhead expelling drops of ink as it travels. By selectively energizing the resistors as the printhead moves across the media, the ink is expelled in a pattern on the media to form a desired image (e.g., a picture, chart or text). The nozzles are typically arranged in one or more linear arrays. If more than one linear array is utilized, the linear arrays may be located side-by-side on the printhead, parallel to one another, and substantially perpendicular to the scanning direction. As such, the length of the nozzle arrays defines a print swath or band. That is, if all the nozzles of one array were continually fired as the printhead made one complete traverse through the print zone, a band or swath of ink would appear on the sheet. The height of this band is known as the “swath height” of the printhead, the maximum pattern of ink which can be laid down in a single pass.
The nozzle plate of the printhead may accumulate contaminants, such as fibers, dust, and the like, during the printing process. Such contaminants may adhere to the nozzle plate for various reasons including the presence of ink on the printhead, or because of electrostatic charges that may build up during operation. In addition, excess dried ink may accumulate around the printhead. The accumulation of ink or other contaminants may impair the quality of the output by interfering with the proper application of ink to the printing medium. Also, if color printheads are used, each printhead may have different nozzles which each expel different colors. If ink accumulates on the nozzle plate, a mixing of different colored inks, known as cross-contamination, can result during use. If colors are mixed on the nozzle plate, the quality of the resulting printed product can be affected. Another possible quality problem may result from particles that may form in the ink disposed in the reservoir, the tubing connecting the reservoir to the printhead, or within the printhead itself due to temperature, contamination, storage time, etc. Furthermore, the nozzles of an ink-jet printer can clog, particularly if the printheads are left uncapped for a period of time. For these reasons, it is desirable to service the printhead on a routine basis. Service procedures may include clearing the printhead nozzle plate of contaminants and ink on a routine basis to prevent the build up thereof. This may be accomplished by a service procedure where a printhead expels ink, is brought in contact with a wiper and expels ink again, also referred to as a spit-wipe-spit procedure, or more simply referred to as a wipe procedure. In some printers this wipe procedure is performed at the end of a print job based on certain criteria, for example, the number of drops fired since the last wipe procedure, the time a printhead has been uncapped, upon a user request, when power has first been applied to the printer, etc.
U.S. Pat. No. 5,455,608 describes how a printer may schedule a service procedure on a printhead based on the result of a drop detection step. Before starting a plot the printer performs a drop detection on all printheads present to detect if any nozzles are non-firing, also referred to as a “nozzle out” condition. If a nozzle out condition is detected in a printhead, the printer triggers an automatic process for servicing the malfunctioning printhead to clear or otherwise recover the malfunctioning nozzle.
This process includes a sequence of nozzle recovery procedures of increasing severity. At the end of each procedure a new drop detection test is performed on the printhead to detect if the printhead is fully recovered. If the drop detection test indicates that a nozzle out condition continues to exist, then another servicing procedure is performed. If, after a predetermined number of procedures, the printhead is still not fully recovered (i.e., at least one nozzle is still out), then the user is instructed to replace the printhead or to discontinue the current nozzle check. Thus, a “nozzle health” detection is performed before each print job and service procedures are performed based on a fixed threshold, in this example, at least one nozzle remaining non-functional.
Service procedures such as the wipe procedure are desirable to maintain print quality, but a disadvantage of these procedures is that they consume time and thus have a negative impact on printer throughput and printer productivity. It has been empirically determined that for some printers a servicing action lasting only one second may have a negative impact on throughput of about 2%. This has become more important over time as customers increasingly require shorter printing times and longer printing lengths. Another disadvantage of the service procedures is that they have a negative effect on the long term health of the printhead. For example, the wiping action has a tendency to degrade the nozzle plate by wearing, scratching and/or distorting the surface.